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[brucekaphan]

This post is essentially a continuation of a tangent thread that evolved within Klaus' M49V evaluation; I didn't want to further distract from the focus of that post, so I'm starting a new one here.

I own a Neumann M249 that Klaus dated at "1964-1965." I recently noticed (by random chance) that the last eight feet of the mic to power supply cable (on the microphone end) was microphonic when handled—when not being handled the cable worked beautifully, but since cables can sometimes be jostled during recording, I didn't want to continue to risk ruining a recording so I decided to cut the offending cable off and re-terminate. My studio is small so I'll never miss that 8' of cable in my room.

Following Klaus' detailed instruction on termination, the cable is now working beautifully—microphonics are gone.  However, if I find myself working in other studios, I'm likely to want a longer cable. A few years ago, for a different mic, I purchased a good supply of Gotham 20102 (751) cable—I have plenty left over to build a longer cable. It's double-reussen, and with slight differences in hues, the internal wires follow the same color code and at least appear to be the same gauge as the original Doerffler cable.

I'm thinking that on a rainy day, I'd like to build the spare (longer cable). Does anyone have any thoughts/feelings comparing the original Doerffler to this Gotham cable? Is this Gotham cable as close as I'll be able to come with contemporary cable, to matching quality with the original Doerffler?

Equally pertinent: if I build a spare cable, I'm going to need the appropriate Tuchel 7 pin connectors. Can anyone tell me if these are still being built and if so, where can I find a contemporary source for these? Of course in pursuit of either new contemporary or old stock, it would also be useful to know the actual name or model number for these connectors...

Any help would be appreciated!

Thanks,
Bruce

[Kai]

Cable microphonics of the type you describe usually origin from the screen litz wires have become brittle and broke over the decades.

I can‘t comment on the sound, but functionally I don’t see a problem using the Gotham cable.
The only thing to watch for is, when changing the length of a tube mic cable, the heater voltage needs to be re-adjusted.
Measurement is done on the mic side BTW.

A bit of under-voltage (longer cable) is less critical than any over-voltage.
Therefore, running a tube mic with varying cables is a bit uncomfortable.


The only source for the 7-pin Large Tuchel plugs are specialists shops and the used market.

The plug is simply called Large Tuchel 7-pin, the one with the thin round pins.
The number is Tuchel (HF) T-3468 (male) and (HF) T-3469 (female) or (HF) NT-3469 (female with swivel stand mount).
A version without HF shield is T-3460 and T-3461 / NT-3461 (swivel).
The HF shield is of minor importance, an active cellphone close to the mic will spill into it any way.


I’ve never seen the version with the larger squared pins (like used in the U-47) in a 7-pin variant, only 3- and 6-pin, so Large Tuchel 7-pin should be the key information.

[brucekaphan]

Kai,

Thanks so much for this thorough reply to my questions. I'm now intrigued by your comment on adjusting the heater voltage. I was unaware that changing cable length would require such an adjustment, and I've never done anything of this nature before. Is making this adjustment something you'd be willing to explain in enough detail so that a relative novice at repairing mics wouldn't stand to do damage?

[Kai]

Maybe Klaus has a better method, here’s how I do it:

PREPARATIONS:
• Arrange everything on a table with non-conductive surface: plastic, glass, dry clean wood - NO METAL!
• Unplug mains power at the wall outlet.
• Plug the mic to the PSU - this way the PSU is fully discharged and safe to open.
• Remove and open the female cable connector, the one that goes into the mic.
• Locate the heater voltage control of your PSU.
Usually you have to open the PSU for that, but some have a screwdriver hole opening.
NOT the stepped, course mains voltage adjust for 110-127-220 V, NOT, really!
• Have a fitting, fully ISOLATED small blade screwdriver handy. The ones electricians use for terminal strips, or the fully plastic ones for electronics adjustments.
• Plug the connector back to the mic.
• Connect a DC voltmeter between pin 4 and 7. Use small clips. Double check the meter clips don’t short with neighboring pins. Then double check again.
• Set meter range to read 5 V DC and switch on the meter.
———————
ADJUSTMENT:
• Get everybody out of the room, even pets.
• Put one hand in your back pocket and leave it there through the whole process.
 This is a good safety measure to prevent being electrocuted when something goes wrong.
• Plug in mains.
• After a minute the meter should read a value close to 4 V. Else you’re measuring the wrong pins.
• Adjust to 3.9 - 4.0 V. Turn slowly and watch for changes.
No changes -> wrong control.
• Unplug mains at the wall outlet.

If something doesn’t work as expected, start again with “Unplug mains at the wall outlet”, before locating the error.

[brucekaphan]

Kai,

Thanks again for your help! I especially like all the reminders to not electrocute myself! I have all-day client-attended sessions the next two days, so it will be at least a few days before I'm able to follow your instructions, but in the meantime, I just wanted to thank you for taking the time to educate me and anyone else who might be looking at the same situation.

Bruce

[soapfoot]

I'd just like to add in case it's not 1000% clear--even if there's a convenient place to measure filament voltage at the mic PSU, you only want to measure at the tube itself, because everything in-between has resistance which will cause voltage drop.

Also, be very careful to protect not only yourself, but the rare tube inside. If you end up damaging the rare, pricey, needs-to-be-selected tube, it will be a very, very annoying and expensive fix.

That's why it's important to ensure the heater voltage is correct as soon as is practical. 

[Kai]

--even if there's a convenient place to measure filament voltage at the mic PSU,..

I definitely didn’t say that.

... you only want to measure at the tube itself, because everything in-between has resistance which will cause voltage drop.

Also, be very careful to protect not only yourself, but the rare tube inside.

This is why I do NOT advocate to open the mic, instead open the cable connector at the mic’s side to measure.
Voltage drop is along the cable’s resistance only, not between mic side connector and tube.
There’s nothing more in between where a significant voltage drop can appear.

This is why, at the PSU side, Neumann says to set filament Voltage to 4.2 V, with an average cable you can estimate ca. 0.2 V voltage drop.
But we don’t want to estimate.

By no means measure at the tube pins!
Any force on the tube pins can cause a leak and destroy the vacuum inside the tube.


BTW: There‘s a valid way to measure filament voltage at the PSU side with sufficient precision:
• Unplug the mic cable at both ends.
• Insert a wire bridge into pin(holes) 4 to 7 female connector.
• Measure the cable resistant at pin 4 to 7 male connector.
• For each Ohm of measured resistance (typically 1-2 Ohm) add 0.1 V to the 4 V adjustment voltage (filament current is 100 mA).

Example: The measured roundtrip cable resistance is 2.0 Ohm.
2.0 x 0.1 V = 0.2 V
4 V + 0.2 V = adjust to 4.2 V

[klaus]

By no means measure at the tube pins!

I would modify that statement:

1. There is no reason not to measure at the tube socket pins in microphones using plug-in tubes.

2. When measuring voltages on AC701 tubes which do not have pins but use bare filament wires sticking out of the glass of the tube, try to stay away from these wires and measure where the wires are soldered to their (solid) mounting columns.

[brucekaphan]

Thanks to everyone who has weighed in on this procedure!

I make no pretense of being an expert electronics technician. Nevertheless, after more than 40 years as a professional recording engineer, and with a reasonable degree of natural "fixit" ability, I have enough practical experience around electronics that I feel capable of checking and adjusting the voltage as has been described above.

What I do not want to do is make matters worse by accidentally doing something stupid, so to that end, if someone could verify that I'm making a reasonable assumption with respect to the correct place to make the voltage adjustment in the PSU, I'd be grateful.

In the attached photo, I've indicated the only variable resistor (besides the polar pattern selector) I can find in the power supply. Is this (R5) the heater voltage control?

[klaus]

In the Neumann NN48h (pictured), R5 is a trim potentiometer which adjusts the tube heater voltage.

[brucekaphan]

Thanks for confirming!
Much gratitude to everyone who shared their knowledge on this topic! I performed the test as described by Kai and when after warming up, the voltage tested 3.92V, I chose to leave well enough alone.